1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, the widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline "octane" number. The catalytic reforming unit therefore must operate more efficiently at higher severity in order to meet these increasing aromatics and gasoline-octane needs. This trend creates a need for more effective reforming catalysts for application in new and existing process units.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction, thus producing a higher yield of aromatics and a lower output of fuel gas, while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed. The increased sensitivity to feed sulfur of these selective catalysts also is known. However, this dehydrocyclization technology has not been commercialized during the intense and lengthy development period. The extreme catalyst sulfur intolerance is believed to be the principal reason for this delay in commercialization. This catalyst may be deactivated rapidly in an existing reforming unit which previously employed a less-sulfur-sensitive catalyst for conversion of a sulfur-containing feed, since traces of sulfur contamination may remain in the process equipment even after conventional cleanup of the equipment. If the effect of sulfur contamination could be eliminated, existing reforming units could be reassigned for paraffin dehydrocyclization operations as large modern naphtha reforming units are constructed in conjunction with refinery modernizations. Conventional oxidation, reduction and acidizing do not provide the completeness of sulfur removal required. Therefore, an exceptionally effective cleanup method is needed for these existing units as a concomitant to the reforming process for paraffin dehydrocyclization.
3. Related Art
Techniques are known in the art for avoiding deactivation of reforming catalysts by sulfur oxides produced from sulfur scale on the equipment during catalyst regeneration. U.S. Pat. No. 2,873,176 (Hengstebeck) discloses avoidance of an oxidizing atmosphere in equipment, other than reactors, which has been exposed to sulfur in the feedstock in order to avoid injury to the catalyst. U.S. Pat. No. 3,137,646 (Capsuto) teaches purging of sulfur from the lead heater of a catalytic reforming unit to the heater stock until SO.sub.2 is not detected to avoid deterioration of the catalyst. U.S. Pat. No. 4,507,397 (Buss) reveals that controlling the water content of a regenerating gas to no more than 0.1 mol % in a catalytic reforming unit having sulfur-contaminated vessels avoids reaction of sulfur oxides with the catalyst. The above patents relate to protecting a reforming catalyst from sulfur scale during regeneration, in contrast to the present invention which addresses the need to remove contaminants evolved during process operation.
U.S. Pat. No. 4,155,836 (Collins, et al.) discloses that sulfur-contaminated reforming catalyst may have its activity restored by discontinuing the hydrocarbon feed and passing hydrogen and halogen over the catalyst to reduce its sulfur concentration. U.S. Pat. No. 4,456,527 (Buss, et al.) teaches that a variety of sulfur-removal options may be used to reduce the sulfur content of a hydrocarbon feed to as low as 50 parts per billion for dehydrocyclization over a catalyst with high sulfur sensitivity. Buss, et al. thus recognizes the need for exceedingly low sulfur to a reforming catalyst selective for dehydrocyclization. Neither of the above references, however, contemplates the use of a hydrocarbon solvent to purge contaminants from a prior-contaminated conversion system. U.S. Pat. No. 3,732,123 (Stolfa et al.) teaches a method of descaling a heater contaminated with sulfurous and nitrogenous compounds by alternate oxidation and reduction techniques. U.S. Pat. No. 4,940,532 (Peer et al.) discloses the use and replacement of a sacrificial particulate bed to remove contaminants from a catalytic-reforming system. Peer does not contemplate the combination of purging contaminants from the equipment of a conversion system using a hydrocarbon solvent and subsequently using a contaminant-sensitive catalyst for hydrocarbon conversion, however.